So I was thinking of getting a smaller bag, don't want to spend lots.....any thoughts (doesnt have to be a change bag specifically as long as I can put a pound land travel change mat in there quite happily). Could I get away with a kids rucksack or something like that at this age (6 weeks)

Can't help with the smaller bag but you are right about still needing the bababing for later. I have one and it started full and then got really empty until we started weaning. Now at 15 months there is milk, water, snacks, toys, clothes. It's rammed!